With the advancement of integration and capacity increase of a large scale integrated circuit (LSI) in recent years, a semiconductor device has been increasingly required to have a smaller width of a circuit line. In the semiconductor device, a desired circuit pattern is formed by a lithography technique, which employs a pattern transfer using an original pattern called a mask (a reticle). A highly accurate mask for use in the pattern transfer is manufactured by using a charged particle beam drawing apparatus having excellent resolutions.
As an example of the charged particle beam drawing apparatus, a charged particle beam drawing apparatus configured to draw a pattern in the following manner has been developed. Specifically, the pattern to be drawn on a workpiece such as a mask or a mask blank is divided into multiple stripe regions, and each stripe region is divided into a large number of sub-regions. While a stage having the workpiece placed thereon is being moved in a longitudinal direction of the stripe region, an electron beam is positioned in one of the sub-regions by using main deflection and is shot at predetermined positions by using sub-deflection.
In such a charged particle beam drawing apparatus, a phenomenon called beam drift may occur in which an irradiation position of the electron beam during the drawing is shifted over time due to various factors. One of the factors of the beam drift is generation of an electric field due to reflected electrons. Specifically, reflected electrons are generated due to irradiation of the workpiece with the electron beam, charge-up occurs due to collision of the generated reflected electrons with an optical system, a detector or the like in the apparatus, and thereby a new electric field is generated. The electric field changes an orbit of the electron beam, and thus the beam drift occurs.
In order to cancel out the beam drift, a drift correction is performed. In the drift correction, a drift amount of the electron beam is measured, and a shot position of the electron beam, that is, the irradiation position is corrected so that the drift amount can be cancelled out. In this correction, a mark for position measurement provided on the stage is regularly scanned by the electron beam in order to measure the drift amount. The mark is a mark member having a predetermined height and is fixed on the stage.
Meanwhile, workpieces generally have a tolerance and thus have thicknesses different from each other. For this reason, the height of a workpiece surface on the stage varies depending on the workpiece. Such variation in the height of the workpiece surface causes a shift of the irradiation position of the electron beam. Hence, in order to correct the irradiation position according to the height of a workpiece, some of the charged particle beam drawing apparatuses are proposed each including, for example, a height measurement device configured to measure the height of the workpiece by use of laser light made incident on and reflected from the workpiece surface on the stage, a calibration block configured to calibrate the height measurement device, and the like.
However, the thickness of the workpiece depends on the workpiece. When the drift correction is performed by using the mark as described above, it is difficult to make the workpiece surface and the mark surface flush with each other, and thus the workpiece surface and the mark surface are different in height in a current situation. For this reason, when the electron beam is made incident obliquely on the workpiece and the mark surface, an error occurs between a drift amount on the mark surface and a real drift amount on the workpiece surface. However, the drift amount on the mark surface is used for the drift correction without any adjustment. This leads to a drift correction inadequate due to the aforementioned error, and the inadequate drift correction causes a shift of the irradiation position of the electron beam and therefore lowers the pattern drawing accuracy.
In particular, when the aforementioned electric field generated due to the charge-up caused by the reflected electrons abruptly disappears, an angle of incidence of the electron beam is abruptly and largely changed. The larger the angle of incidence of the electron beam, the more considerable the error between the drift amount on the mark surface and the real drift amount on the workpiece surface. This also leads to the inadequate drift correction as in the case described above, and the inadequate drift correction causes a shift of the irradiation position of the electron beam and therefore lowers the pattern drawing accuracy.